The Stability of Anodized Titanium Surfaces in Hot Water

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Abstract
Adhesive bonding of titanium structures for high performance aircraft has gained increased attention in recent years, but very little work has been done to determine the stability of bond strength in humid environments or the effect of higher operating temperatures envisioned in future aircraft. In this paper, the stability of an important bond component–the surface oxide– is considered. It is shown that an anodically formed amorphous TiO2 layer on Ti-6A1-4V is unstable in pure water at 85°C. Morphological and structural changes in the oxide and possible corrosion of the substrate lead to the formation of an anatase crystallite layer and an increase in the surface oxide mass. Crystallization does not occur when the oxide is heated in vacuum at typical adhesive curing temperatures. The relevance of oxide instability to bond failure is discussed.